Demographics and Monitoring of Repatriated Razorback Suckers in Lake Mohave

Monitoring of repatriated razorback suckers (Xyrauchen texanus) in Lake Mohave has been conducted for more than 20 years, but low recapture rates have inhibited the evaluation of factors contributing to highly variable post-stocking survival. To increase the number of encounters with marked fish, deployment of remote passive integrated transponder (PIT) scanners able to detect 134.2-kilohertz (kHz) PIT tags was initiated in 2011, and expanded in 2012 and 2013, while traditional capture methods were employed to continue to collect comparable long-term monitoring data and estimate abundance of all repatriated and wild razorback suckers marked with either 400- or 134.2-kHz PIT tags.

Trammel netting efforts from October 1, 2011, to September 30, 2014, resulted in the capture of 94 razorback suckers. Sixty-four percent (%) (n = 60) of captures occurred during March “round-up” events and 36% (n = 34) during November routine monitoring. PIT tags were not detected in seven fish presumed to be repatriates that lost their tags, and one wild individual was collected during March 2014 monitoring; all remaining individuals were PIT-tagged repatriates. Based on monitoring data from the last 3 years, we estimate that there is no effective wild razorback sucker population remaining in Lake Mohave. The repatriated razorback sucker population in 2011, based on 2011 and 2012 monitoring data, was estimated to number 2,577 (95% confidence interval [CI] from 1,139 to 6,284). In 2012, repatriate population estimates from March data declined by over 700 individuals, totaling 1,854 (95% CI from 941 to 3,782). The current (2013) total population estimate for razorback suckers in Lake Mohave using March data is 2,525 (95% CI from 1,180 to 5,741).

Total deployment time for remote PIT scanners from October 2011 through September 2012 was 8,330 scan hours, resulting in 46,855 PIT tag contacts, representing 2,748 unique PIT tags for which 2,710 had a razorback sucker marking record in the Lower Colorado River Native Fish Database (as of August 1, 2014). Among fish with a marking record, 2,698 were repatriates, and 12 were wild. During the 2013 sampling season, remote antennas scanned for a total of 11,426 hours, recording 197,149 PIT tag contacts, more than four times as many as were observed in 2012. Of these total contacts, 3,222 were unique PIT tags, 3,147 of which had a marking history in the Lower Colorado River Native Fish Database. Almost all were repatriates (3,137), and 10 were recorded as wild individuals. Scanners were deployed for 8,955 hours from October 2013 through September 2014, almost 2,500 fewer hours than 2013, but resulted in 239,170 contacts, the most over the past three sampling seasons. These contacts represented 2,709 unique PIT tags for which 2,632 had a marking record in the Lower Colorado River Native Fish Database. Among razorback suckers with a marking record, 2,621 were repatriates, and 11 were wild.

Lake Mohave was subdivided into stocking zones, and up- to downstream, these were: river, Liberty, basin, and Katherine. Post-stocking dispersal from zone to zone over the course of the study period was limited. The majority (> 90%) of fish released in the river and basin zones were contacted in their zone of release regardless of release year. Razorback suckers released in the Liberty zone were generally contacted elsewhere (river and basin zones); however, these fish accounted for 6.9% of the total number of fish contacted in only one zone (113 of 1,619). Remote PIT scanning detected little movement of razorback suckers among the three zones scanned in 2012 and 2013 (river, Liberty, and basin), with 92.4% of individuals (1,561 out of 1,689) contacted in the same zone both years. The same post-stocking dispersal trends were observed in 2013 and 2014; fish released in the river and basin zones tended to stay there, and fish released in the Liberty zone were contacted either up- or downstream. Individuals contacted in both 2013 and 2014 also exhibited minimal movement; more than 91% of the fish (1,528 of 1,674) were scanned in the same zone from year to year. The Katherine zone had too few stockings and captures and no scanning to evaluate dispersal.

Based on 2011 and 2012 remote PIT scanning, the 134.2-kHz tagged Lake Mohave repatriate population was estimated at 2,704 (95% CI from 2,437 to 3,001). Subpopulation estimates based on zone-specific scanning in 2011 and 2012 were also calculated. The basin zone population was estimated at 948 (95% CI from 795 to 1,130), and the river zone population was estimated at 1,851 (95% CI from 1,623 to2,111). Using 2012 and 2013 scanning data, Lake Mohave tagged repatriates were estimated to number 3,447 (95% CI from 3,142 to 3,783). Zone-specific scanning resulted in a subpopulation estimate for the basin zone of 1,509 (95% CI from 1,324 to 1,718), in the Liberty zone at 44 (95% CI from 13 to 80), and in the river zone at 2,169 (95% CI from 1,892 to 2,486). The river zone estimate was nearly identical to an estimate of 2,174 from a regression analysis conducted in 2012. With 2013 and 2014 PIT scanning contacts, the Lake Mohave population was estimated at 3,284 individuals (95% CI from 3,067 to 3,516). The basin zone subpopulation was estimated at 1,492 (95% CI from 1,357 to 1,640) and in the river zone at 2,053 (95% CI from 1,853 to 2,275). Subpopulations in the Liberty zone were not estimated in 2012 or 2014 due to a lack of scanning effort there during the sampling season. Although wild fish also were contacted in the basin and river zones, no estimate was calculated because a limited number of recaptures were recorded. Too few data were available to support a population estimate for the Katherine zone.

Based on a multi-strata mark-recapture model assessment in the computer program MARK, an estimated 6.0% (95% CI from 4.4 to 8.1%) of razorback suckers transitioned from the basin to the river zone post-release to January 2012 and 6.6% (95% CI from 4.3 to 10.0%) from the river to the basin zone. Monthly transition rates for adult razorback suckers were estimated at 0.6% (95% CI from 0.4 to 0.9%) for razorback suckers moving from the river to the basin zone and 2.3% (95% CI from 1.8 to 2.9%) from the basin to the river zone. Monthly survival was estimated at 99% for most of the year, declining for 1 or 2 months each year between January and April, with estimates as low as 74.1% (excluding parameter estimates that were confounded with recapture rates).

Biannual netting efforts should continue in order to collect growth, health, census, and genetic data for razorback suckers. Until an effective method to collect larvae upstream of Willow Beach is discovered, future repatriation efforts should be focused in the basin zone. Remote PIT scanners should be deployed to monitor the two known subpopulation centers (river and basin zones) with a nominal effort of 200 scanning hours per zone. Additional effort should be distributed throughout Lake Mohave in an effort to determine if other aggregations exist.

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